Wheeled vehicles, such as cars, all-terrain vehicles (ATVs), three-wheeled vehicles and motorcycles, are generally equipped with one or more brakes for slowing or stopping the vehicle. The driver of the vehicle can actuate the brakes by actuating a pedal, lever, or other actuator that is positioned on the vehicle within his reach. One common type of brake, a disk brake, uses friction between a brake caliper and a brake disk to slow or stop the rotation of a wheel of the vehicle relative to the body of the vehicle. The resulting friction between the tire and the terrain exerts a braking force on the vehicle to slow the vehicle.
In general, it is desirable to be able to minimize the distance traveled by a moving vehicle before it stops. To this end, it is desirable to maximize the friction between each wheel and the terrain during braking. It is well known that the maximum available friction that can be generated by a tire when slipping with respect to terrain (kinetic friction) is lower than the maximum available friction that can be generated by the tire when rolling without slipping with respect to the terrain (static friction). Therefore, braking performance is improved on most types of terrain by increasing the braking torque on the wheels, up to the point where the braking force between the tire and the terrain is barely insufficient to cause slipping. One application of this principle is threshold braking, in which the driver modulates the brakes to provide as much braking torque as possible before the tires begin to slip. However, effective threshold braking depends on the skills and experience of the driver, and can be difficult to perform on some types of terrain. In addition, threshold braking does not allow the braking force at each wheel to be independently controlled, which may be desired when not all of the wheels are on the same type of terrain (e.g. some wheels on dry pavement and other wheels on ice), and some wheels may begin slipping before others achieve their maximum braking force.
One attempt to improve braking performance is with the use of an anti-lock braking system (ABS). A control unit detects differences in rotational speed between the wheels of the vehicle to determine whether one or more tires is slipping with respect to the terrain. If a particular tire is slipping, the control unit decreases the braking torque on the corresponding wheel in an attempt to restore static friction and maximize the braking force generated by the wheels. ABS essentially performs threshold braking individually for each wheel. As a result, each wheel independently rotates at the speed that provides the most braking force on its particular terrain, and therefore contributes as much as possible to the braking of the vehicle. An added benefit of ABS is that the car can be steered while braking, because the wheels do not lock and maintain some traction.
While ABS is now commonly used in cars, its cost is often prohibitive in relation to the price of an ATV. In addition, the components, in particular the brakes on each wheel, tend to both increase and decentralize the weight of an ATV, whereas minimizing weight is an important design consideration for ATVs. In addition, individual brakes constitute unsprung weight, which can decrease ride quality. In addition, ABS is limited in its applicability to two wheels joined by a solid axle or a locked differential, such as some ATVs, because the two wheels would not be permitted to rotate at different speeds when this would be desired in certain braking conditions.
In order to minimize and centralize vehicle weight, some ATVs are provided with a single rear disk brake, either on a portion of a solid rear axle, or on a driveshaft extending from the engine to a rear differential. An example of such an arrangement is described in U.S. Pat. No. 6,491,126, issued Dec. 10, 2002, which is incorporated by reference herein in its entirety.
Therefore, there is a need for a braking system for an ATV that allows for differential rotational speeds of the wheels to which it is applied.
There is also a need for a method of braking a vehicle that provides differential rotational speeds of the wheels to which it is applied.